Bone Charms are small objects that slightly modify the abilities of Corvo. You will first encounter them once you reach the Hound Pits. They can be located by equipping the Heart tool, just like you would a Rune. There are over thirty known Bone Charms currently in the game, so if you have missed a few of them you will have plenty of opportunities to find more.
If you purchased a pre-order pack you will automatically be given three Bone Charms at the start of the game. These count towards the trophy and will allow you to unlock it at the start of Mission 3: House of Pleasure, once you visit Slackjaw in the Distillery—if you have collected all of the available Bone Charms up to that point. For those that do not have the bonus Bone Charms you will be able to unlock the trophy no earlier than the very end of Mission 3.
Collecting 10 over the course of the game is not difficult at all. The Heart precisely shows you where each Bone Charm is in a level, and you will have many chances to collect them as the missions are generally open ended.
This trophy is technically missable as you can complete the game without collecting any Bone Charms, but they are so easy to find that missing out on this trophy is not very likely, although still possible.